It is generally known that when soaked steel ingots are primary-rolled into semifinished products called blooms, slabs and billets, various types of cracking occur depending upon such factors as the composition of the ingot, its structure, heating profile and rolling conditions. A transversal cracking phenomenon, which develops in steel ingots during the rolling process, is characteristic in aluminum deoxidized low- and medium-carbon steels and low-alloy steels; transversal cracking is detrimental to rolling operations and reduces their yield so much as to cause great economic loss.
Steel ingots withdrawn from molds are first heated in a heating furnace to the proper temperature for primary rolling, and from the viewpoint of steel manufacture process, the withdrawn ingot is subjected to one of the following conditions before it is charged into the heating furnace:
(i) neither the interior nor the surface area of the ingot cools down excessively but they are held at a temperature significantly higher than the Ar.sub.3 transformation point while the ingot is charged into the heating furnace; PA1 (ii) the temperature of the ingot in its surface area has lowered just above the Ar.sub.3 transformation point, namely, the overall ingot temperature is lower than in the above conditions (i) but the ingot is still austenitic while it is charged into the heating furnace; or PA1 (iii) the withdrawn ingot is cooled slowly so that not only its surface area but also the whole ingot cools down below the Ar.sub.1 transformation point, sometimes close to ordinary temperatures, and the thus cooled ingot which is generally referred to as a "cold ingot" is charged into the heating furnace where it is heated to the rolling temperature. PA1 (1) cooling a bloom produced by continuous casting or a steel ingot produced with a mold to bring the surface temperature thereof to 50.degree. to 150.degree. C. higher than the Ar.sub.3 transformation point thereof; PA1 (2) quenching the bloom or the steel ingot in such a way that its interior remains red hot while the surface area is transformed to have a bainite structure; and PA1 (3) heating the bloom or the steel ingot in a furnace followed by hot shaping.
Steel ingots to be rolled are charged into the heating furnace under one of these three conditions depending upon the weight and shape of the ingot, the temperature at which it is withdrawn from the mold, and the time to charging into the heating furnace. The risk of cracking is small during the rolling of the ingot that has been charged into the heating furnace in accordance with the conditions (i); on the other hand, almost all reported cases of transversal cracking are associated with the conditions (ii). No case has been known that transversal cracking occurred during the rolling of ingots treated under the conditions (iii).
The above shows both statistically and experimentally that the occurrence of transversal cracking depends on the manner in which ingots withdrawn from the mold are charged into the heating furnace. In other words, the transversal cracking of ingots is the least associated with factors in steel manufacturing, heating and rolling processes but is governed most by the profile of temperature drop which the ingot experiences after it is withdrawn from the mold and before it is charged into the heating furnace.
JP-B-49-7771 discloses, as a result of the above finding concerning the operation of rolling steel ingots, a method of hot working a steel ingot, in which the ingot is immersed in a circulating coolant in a vessel or sprayed with a propelled coolant in such a rapid manner that the interior of the ingot remains red hot while only its surface layer is cooled down below the A.sub.1 transformation temperature and, thereafter, the ingot is heated in a furnace followed by hot shaping. (The term "JP-B" used herein means an examined Japanese patent publication.)
The transversal cracking of ingots is caused either by the extreme coarsening of columnar crystals in the cast structure of the ingot surface layer during heating, or by the fracture which occurs during primary rolling in the surface area of the ingot which has become brittle due to the oxidation of the grain boundaries of austenite crystals in the surface area. Noting the above facts, in the method of JP-B-49-7771, only the surface layer of the ingot is quenched, so that its columnar crystallographic structure is divided into fine portions while the grains of austenite crystals which form in subsequent heating are refined.
In the case of aluminum killed steels, dissolved aluminum binds with the nitrogen in the steel to form aluminum nitride. If its production exceeds the solubility limit in the course of temperature drop following the solidification of the ingot, the aluminum nitride is deposited as a tabular precipitate at austenite grain boundaries, eventually causing surface cracking. Under the circumstances, the surface layer of the ingot is quenched so that the precipitation of aluminum nitride at austenite grain boundaries is sufficiently suppressed to prevent transferal cracking.
The method described in JP-B-49-7771 is very effective in the case of producing steel ingots with ordinary molds, since the surface temperature of the ingot for starting the quenching can be freely selected so that it can be quenched from comparatively high temperatures. However, this is not the case for producing blooms by a continuous casting machine. When molten steel comes into contact with a water-cooled mold, the cooling action of the mold causes a thin solidified skin to form on the surface. In order to prevent the solidified skin from rupturing caused by withdrawing the casting from the mold by means of pinch rolls which are positioned below, the cast bloom must be cooled more rapidly than the ordinary ingots. Therefore, in the process of continuous casting, the temperature difference between the surface and the interior of the casting is so great as to increase the chance of the occurrence of strains such as transformational strains. In addition, strain due to the ferro-static pressure of molten steel and the external strain caused by straightening rolls will also act on the continuous-cast bloom, thereby causing cracks to develop more frequently than in the case of the ordinary cast ingots. Under the circumstances, the continuous casting process requires positive cooling of the surface of a solidifying steel bloom but its temperature thus drops just above the Ar.sub.3 point, which has made it impossible to fully attain the advantages of the method described in JP-B-49-7771.
JP-A-63-168260 proposes a method for solving the aforementioned problems associated with the production of blooms by a continuous casting machine. (The term "JP-A" as used herein means an unexamined published Japanese patent application.) JP-A-63-168260 discloses a method of hot working a continuous-cast bloom, in which a killed steel bloom produced by continuous casting is first cooled to bring its surface temperature to 150.degree. to 50.degree. C. higher than the Ar.sub.3 transformation point, then quenched with a cooling medium in such a way that the interior of the bloom remains red hot while the surface temperature becomes 100.degree. to 400.degree. C. lower than the Ar.sub.1 transformation point and, thereafter, the bloom is cut to predetermined lengths, which are subsequently heated in a furnace followed by hot shaping.
This method is characterized in that when the bloom immediately after cast in a continuous uncut form is still hot on the surface and has a specified surface temperature higher than the Ar.sub.3 transformation point where the bloom is solely composed of an austenite structure, the surface layer of the bloom is quenched by a suitable method such as water spraying. This method is capable of effectively suppressing the surface cracking that develops in continuous-cast blooms.
Recently, in order to prevent the coarsening of crystal grains during carburization, low-alloy steels and low-carbon steels that are especially adapted for carburization through positive addition of nitrogen have recently come to be produced in increased quantities. These steels generally contain from 0.0080 to 0.0300% by weight of nitrogen to have high aluminum nitride contents, and therefore they are highly susceptible to cracking at elevated temperatures and have suffered from the problem of frequent surface cracking during hot working.
Furthermore, with the recent increase in demand for steels of good cuttability, free-cutting steels containing lead have come to be produced in increased quantities. Such steels generally contain from 0.03 to 0.25% by weight of lead, and since the lead causes adverse effects on hot workability at elevated temperatures, they suffer from the same problems as the nitrogen-containing steels and experience frequent surface cracking during hot working.
The above two prior art methods described in JP-B-49-7771 and JP-A-63-168260 have proved to be very effective for the purpose of suppressing the occurrence of surface cracking in many species of steels. However, they are not as effective on the nitrogen- or lead-containing steels which have seen increasing use these days and a need has arisen to develop an improved production process.